1. Field of the Invention
The invention relates to a method and device for controlling the pressure in a vented single-screw extruder or a cascade extruder.
2. Description of the Background
The extrusion of thermoplastics to form shaped bodies, such as sheets or films, generally takes place using a vented single-screw extruder if volatile constituents, which have an adverse effect on the properties of the extrudate which is to be produced, are to be separated out during the extrusion process. These volatile constituents may, for example, be water, monomers or oligomers. In the vented single-screw extruder, the plastic which is in granule form is initially melted, compressed and homogenized in a first screw section, comprising a feed section, a compression zone and a first homogenization zone (referred to below as the introduction zone) and is then fed to a second screw section, which comprises a low-cut venting zone and a metering section, in which the melt is vented, compressed and conveyed into an extrusion die, from which the melt then emerges in the desired form.
An alternative design is what is known as the cascade extruder. In this arrangement, the screw is divided, in accordance with the abovementioned two screw sections, into two successive single screws with separate drives. The rotational speeds of the two screws are usually kept in a fixed ratio.
Extruders of the type described are usually equipped with a temperature-control system which allows heat to be supplied or dissipated via the barrel, in order to set the temperature in the region of the screw in the desired way. The temperature can be controlled by means of a heat transfer liquid, the temperature of which is in turn controlled by means of known heating/cooling equipment. Combinations of electrical heating and air or water cooling are often also used. Controlling the temperature by means of a thermocouple in the barrel, which actuates a controller which in turn initiates the supply or dissipation of heat, forms part of the prior art.
To allow temperature control which is as well matched to the demands of the process as possible, the temperature control over the length of the extruder is usually divided into a greater number of independently controllable zones.
DE-C 27 58 265 describes a control device for monitoring the filling level of a venting opening at an extruder for processing molten low-density polyethylene.
DE-C 37 44 193 describes a method for venting a thermo-plastic melt over a wide viscosity range using a plurality of venting stages which are connected in series.
Stehr, R. and Andersen, P., in ANTEC '92, pp. 416–420: “Controlling a Gear Pump Assisted Degassing Extruder via Dual Pressure Sensing”, describe a device comprising a vented single-screw extruder with connected melt gear pump. The vented single-screw extruder with first and second screw sections has a measurement control circuit for maintaining a predetermined pressure at the end of the metering section, the intention being to ensure that the corresponding extruder zone is filled with melt but no melt emerges from the venting opening. The pressure diagrams show high pressures, around 30 bar, for the first half of the metering section (pm). The pressures downstream of the screw tip (ps) are shown to be lower and apparently undergo considerable periodic fluctuations in the range between 15 and 30 bar. There is no suggestion that the pressure be controlled in the first half of the metering section, with the aim of avoiding partial filling with melt there.
A critical point in designing the screw geometry of a vented single-screw extruder and in defining the ratio of the screw rotational speeds in a cascade extruder is that of matching the transport behaviour of the two screw sections. In doing so, it is necessary to ensure that in any operating state of the extruder (plastic, output and temperature), the second section can transport away more melt than the first section supplies.
Otherwise, melt emerges from the venting section, which generally causes the process to fail (the installation has to be stopped in order to clean the venting section and then started up again). To ensure that the extruder can be used over a wide operating range, the transport behaviour of the two sections is usually set in such a way that the second section has considerably better transport properties than the first section, so that even in critical operating states it is reliably possible to prevent melt from escaping from the venting section. This means that it is often the case that not only the low-cut venting zone, but also a part of the metering section which is of some length are only partially filled, i.e. melt is located only directly ahead of the sliding flank of the screw flight. Consequently, plastic can accumulate in the regions which are not permanently filled with melt. Particularly if irregularities occur in the transporting of the melt during extrusion, it may be the case that partial quantities of the melt remain in the extruder for an excessively long time, where they are exposed to the high temperatures produced in contact with the inner surface of the screw. This may cause some of the melt to be converted into a gel-like mass or even, in the event of relatively unfavourable conditions, into carbon-like deposits. These deposits are to some extent detached again during extrusion and as a result pass into the melt as undesired impurities.
The problems described occur both while the extrusion process is ongoing, as a result of fluctuations in the transport behaviour of the first screw section, and also, in particular, in the event of considerable changes in the operating point, as caused by a change in the output or in the plastic which is being processed.
Moreover, the problems which have been described mean that, when the processed plastic is changed, if the two plastics are incompatible, it is imperative that the screw be dismantled and cleaned, which is a complex process. Otherwise, impurities caused by the plastic which was previously processed will occur even a relatively long time after the plastic has been changed.
Only the carbon-like deposits which were described above can be at least partially separated out by using standard melt-filtration systems. Removal of gel-like impurities or of other plastics with similar processing temperatures is not possible.
Particularly transparent products, with high demands imposed on the visual properties, can be made unusable by these impurities.
Generally, a melt pump for keeping the flow rate constant and filter units for separating impurities out of the melt are located between the vented single-screw extruder or the cascade extruder and the extrusion die.
To match the flow rates of extruder and melt pump, the pressure between the extruder tip and the melt pump is usually measured as a control variable and is regulated to a predetermined set value by means of the extruder screw rotational speed as adjustable value (cf. for example the Konrad Kerres dissertation, Augustinus Verlag, Aachen, 1995). This control arrangement is used to compensate for changes in the transport behaviour of the extruder, for example caused by the temperature in the feed section or changes in the characteristics of the plastic (apparent density, feed behaviour).
A measure of the uniformity of the melt transport in the melt pump is the pressure downstream of the pump. The aim of combining the extruder and melt pump is to limit the pressure fluctuations downstream of the pump to less than ±5%. This aim can be achieved if the pressure upstream of the gear pump changes by no more than ±20%. Greater fluctuations in the pressure upstream of the pump have effects on the uniformity of transport and therefore the pressure downstream of the pump.
The fluctuations in the pressure upstream of the pump are usually compensated for by the above-described control of the rotational speed of the extruder screw. In the case of vented single-screw extruders or cascade extruders with an essentially only partially filled second metering section, however, relatively high-frequency pressure fluctuations may occur and cannot be eliminated by control means. The pressure fluctuates with the frequency of the rotational speed of the screw. On the one hand, this disrupts the above-described control of the pressure upstream of the pump and, on the other hand, causes fluctuations in the flow rate, which may have effects on the geometry of the extrudate produced.
The set value for the pressure upstream of the melt pump is usually input by the machine operator in such a way that, on the one hand, the pump provides a build-up of pressure (this is imperative on account of the design principle of most pumps, which use the transported melt as lubricant), while on the other hand a maximum pressure, which results from the permissible locking forces at the flange between extruder and melt pump, must not be exceeded.